When agriculturally active chemicals (agrochemicals) are relatively water soluble, preparing, storing, and shipping the same in a commercially acceptable form can be relatively clear-cut. However, many agrochemicals are hydrophobic and formulators are often confronted with difficulties in finding a suitable means for preparing these materials in stable formulations that deliver maximum loading of active ingredient per unit volume to the end-user.
A straight-forward approach to preparing concentrated liquid formulations with agrochemicals having limited aqueous solubility has been through the use of aromatic organic solvent systems. In such systems, aromatic organic solvents such as xylene or kerosene are used to solubilize the agrochemical compound of interest.
Commonly, surfactants are added to the agrochemical-solvent compositions to form emulsifiable concentrates. The surfactant-emulsifiers interact with the agrochemicals in a number of ways both before and during actual use, i.e., application to the site. The surfactants can initially disperse and/or emulsify the agrochemical in the solvent or in an inert carrier media and, for example, with herbicides, the surfactant composition may also act as a penetrant, spreader, sticker, stabilizer, and wetting agent. The surfactant composition may affect the rate of drying of a droplet on a plant and the nature of a residue liquid, or crystal. The surfactants may also influence the weathering characteristics of an agrochemical, including its rewetting characteristics and rainfastness.
Microemulsions are a subclass of emulsions and are typically clear solutions. In general, microemulsions are characterized by emulsion droplets having average particle sizes between 0.01 and 0.1 micron. The small particle size allows for the emulsion to be more stable than an emulsifiable concentrate formulation.
There is a need for microemulsifiable agrochemical concentrates that have a high biological activity in the target application, have good chemical and physical stability under a severe range of conditions that can be experienced in the marketplace, have good ecological and toxicological properties and can be readily water-dilutable to form a microemulsion.
Azoxystrobin, a strobilurin, is a fungicidal compound with a broad spectrum of disease control. Azoxystrobin inhibits mitochondrial respiration by blocking electron transfer between cytochrome b and cytochrome c1, at the ubiquinol oxidising site. Azoxystrobin is a fungicide with protectant, curative, eradicant, translaminar and systemic properties and inhibits spore germination and mycelial growth, and also shows antisporulant activity. At labelled application rates, azoxystrobin controls the numerous pathogens including Erysiphe graminis, Puccinia spp., Lepiosphaeria nodorum, Septoria tritici and Pyrenophora teres on temperate cereals; Pyricularia oryzae and Rhizoctonia solani on rice; Plasmopara viticola and Uncinula necator on vines; Sphaerotheca fuliginea and Pseudoperonospora cubensis on cucurbitaceae; Phytophthora infestans and Alternaria solani on potato and tomato; Mycosphaerella arachidis, Rhizoctonia solani and Sclerotium rolfsii on peanut; Monilinia spp, and Cladosporium carpophilum on peach; Pythium spp. and Rhizoctonia solani on turf; Mycosphaerella spp. on banana; Cladosporium caryigenum on pecan; Elsinoë fawcetii, Colletotrichum spp. and Guignardia citricarpa on citrus; Colletotrichum spp. and Hemileia vastatrix on coffee. Azoxystrobin is a solid material having low solubility in water.
Propiconazole is a steroid demethylation (ergosterol biosynthesis) inhibitor. Propiconazole is a systemic foliar fungicide with protective and curative action, with translocation acropetally in the xylem. At labelled application rates, propiconazole controls numerous diseases caused by, for example, Cochliobolus sativus, Erysiphe graminis, Leptosphaeria nodorum, Puccinia spp., Pyrenophora teres, Pyrenophora tritici-repentis, Rhynchosporium secalis and Septoria spp. on cereals; Mycosphaerella musicola and Mycosphaerella fijienis var. difformis in bananas; Sclerotinia homoeocarpa, Rhizoctonia solani, Puccinia spp., Erysiphe graminis in turf; Rhizoctonia solani, Helminthosporium oryzae and dirty panicle complex in rice; Hemileia vastatrix in coffee; Cercospora spp. in peanuts; Monilinia spp., Podosphaera spp., Sphaerotheca spp. and Tranzschelia spp. in stone fruit; and Helminthosporium spp in maize. Propiconazole is a substantially water-insoluble liquid at 20° C.
Water-dispersible granular formulations of azoxystrobin are currently known for use on turf grass, Microemulsifiable concentrates of azoxystrobin have been prepared and are commercially available under the name of, for example, HERITAGE® TL fungicide available from Syngenta Crop Protection, Inc. (Greensboro, N.C., USA). HERITAGE® TL fungicide is a liquid formulation having many properties characteristic of a microemulsifiable concentrate. Upon dilution, while not truly thermodynamically stable, HERITAGE® TL forms an optically transparent emulsion having average emulsion droplet sizes in the range of 0.01 to 0.1 microns and is stable for extended periods of time, Microemulsifiable concentrates of other fungicides are known as well including microemulsifiable concentrates of propiconazole commercially available under the name of, for example. Banner MAXX® fungicide available from Syngenta Crop Protection, Inc. (Greensboro, N.C., USA). As described above, the microemulsifiable concentrates do provide a number of advantages upon dilution, for example, they mix very easily with water and require minimal agitation. The microemulsions formed are quite stable, the emulsion droplet average particle size is smaller than that of an emulsifiable concentrate and biological activity may be superior.
It is often desirable to use mixtures of fungicides having different modes of action in order to increase the spectrum of fungal disease control as well as a means for resistance management. Mixtures of azoxystrobin and propiconazole are known and include QUILT™ fungicide, a suspoemulsion formulation commercially available from Syngenta Crop Protection, Inc. (Greensboro, N.C., USA). In this suspoemulsion, the azoxystrobin is present as a dispersed solid, while the propiconazole is in a dispersed liquid state.
While microemulsions prepared by diluting microemulsifiable concentrates are relatively stable compared to other formulation types, when the active ingredient starting materials are high melting solids, there is the possibility that these materials can crystallize and fall out of solution over time. This has been observed in microemulsions of azoxystrobin. When initially formed the formulations have an average emulsion droplet size of between 0.01 and 0.1 microns, however, over time the azoxystrobin can re-crystallize and settle. It is one object of the present invention to prepare microemulsifiable concentrates which, upon dilution, form liquid azoxystrobin microemulsion formulations having prolonged physical stability.